1. Field of the Invention
The present invention relates to a cartridge type recording medium incorporating a recording disk in its cartridge, a driving unit performing recording and/or reproducing to the cartridge type recording medium, and the like.
2. Description of the Related Art
Along with the popularization of digital content, disk-shaped recording media such as CDs (compact disks) and DVDs (Digital Versatile Disks) has been widely used as media for housing the digital content in recent years. When a driving unit such as a recording and/or reproducing unit is equipped with such a recording medium, a convex fitting portion formed in the driving unit is coupled into a center hole having a diameter of approximately 15 mm, which is bored at the center portion of the recording medium. Thus, centering (positioning) of the recording medium with respect to the driving unit is executed.
Incidentally, as to the conventional recording medium, a plastic substrate having relatively small rigidity and moreover having a large coefficient of thermal expansion is used, and a hole of a relatively large diameter of 15 mm is formed in the substrate. Therefore, it is difficult to thoroughly position the recording medium relative to the fitting portion of the driving unit when a simple coupling is employed. This is because, in order to position the recording medium relative to the fitting portion thereof with the simple coupling, especially the center hole of the recording medium is required to have high dimensional precision, rigidity and the like for preventing deformation due to contact with the fitting portion of the driving unit and deformation due to temperature change.
Accordingly, it is generally adopted that the conventional recording medium generally comprises a centering member 12 onto the fitting portion 11 of the driving unit 10. The centering member 12 has a tapered shape having an outer diameter that gradually becomes smaller toward the tip end portion thereof, as shown in FIG. 12. When the recording medium 20 is loaded to the driving unit 10 having such fitting portion 11, it is possible that an error and the like in a diameter of the center hole 22 formed in a disk substrate 21 of the recording medium 20 is absorbed and positioning precision is enhanced because of the tapered shape of the centering member 12. Furthermore, the coupling of the recording medium 20 to the fitting portion 11 can be easily executed.
However, in such method, it has been difficult to accurately position the centering member 12 to the fitting portion 11 of the driving unit 10 when flashes, dirt and the like exist on an edge portion 22a of the center hole 22.
Incidentally, it is drastic that these recording media have been improving in higher recording density in recent years. Along with the above, a pitch of tracks in which data is recorded (hereinafter, referred to as a track pitch) tends to become narrower. When the track pitch becomes narrower as written above, positioning of the recording medium to the driving unit is required to be more precise than ever in order to surely record/reproduce data by a pickup of the driving unit.
For example, HD-DVD and the like that are next generation optical recording media of the same size as CD having recording capacity exceeding 20 GB have track pitches of approximately 0.3 μm, and therefore, the positioning precision in eccentricity of the recording medium relative to the driving unit must be 15 μm or less.
However, it is actually obliged to say that positioning of the recording medium 20 to the driving unit 10 is practically impossible with required precision due to the foregoing reasons in the system shown in FIG. 12 to meet such requirements.
Therefore, in the recording medium 30 such as a magneto-optical disk regulated in the ISO (International Organization for Standardization), a structure in which a center hub 34 having a hole 33 of a diameter smaller than that of a center hole 32 formed in a disk substrate 31 is combined in one united body to the disk substrate 31 is adopted as shown in FIG. 13. In this structure, a centering shaft 42 provided in a fitting portion 41 of the driving unit 40 is coupled into the small diameter hole 33, and thus positioning of the recording medium 30 to the driving unit 40 is executed.
With such structure, a material having rigidity higher than that of the disk substrate 31 and a coefficient of thermal expansion smaller than that of the disk substrate 31 is adopted for the center hub 34, and the hole 33 smaller than the center hole 32 is formed. Thus, dimensional precision and dimensional stability of the hole 33 itself can be further enhanced than those of the center hole 32, accordingly it is possible to secure high positioning precision of the recording medium 30 to the driving unit 40.
As described above, from the viewpoint that merely high positioning precision is merely secured, a system in which the center hub 34 having the small diameter hole 33 formed therein as shown in FIG. 13 is preferably adopted for the recording medium spread in the market is in the year to come.
However, recording mediums such as CDs and DVDs have already spread among users in the present. Existing CDs and DVDs adopt a constitution in which positioning is executed by use of the center hole 22 having a diameter of 15 mm, which is formed in the disk substrate 21 (hereinafter, referred to as an old system as the need arises), as shown in FIG. 12.
To the contrary, if a constitution (hereinafter, referred to as a new system as the need arises) in which positioning is executed by use of the small diameter hole 33 formed on the center hub 34 is adopted for the recording medium that will spread from now on, as shown in FIG. 13, compatibility between the old and the new systems is lost, and users have to prepare the old system driving unit 10 for the old system recording medium 20 and the new system driving unit 40 for the new system recording medium 30. Accordingly, it cannot be said that convenience for the users is high with this.